Development of pia‐arachnoidal membranes in the mouse occurs in four stages: the first (prenatal days 10–13) follows closure of the neural tube and is a period of initial vascularization of the developing telencephalon; the second (prenatal days 14–16) is a period of delineation during which the limits of the subarachnoid space are defined; the third (prenatal day 17 to birth) is a period of ensheathment of pia‐arachnoidal blood vessels, and the fourth (birth to postnatal day 21) includes addition of smooth muscle to larger vessels, the appearance of macrophages in the subarachnoid space, and a general increase in extracellular collagenous and elastic fibers. The mesenchyme over the telencephalic surface in the 10‐day fetus has a typically large extracellular space. By the 13th fetal day cerebrospinal fluid begins to seep into and replace it. The mesenchymal extracellular compartment is reduced peripherally, resulting in a compacted pia‐arachnoidal tissue which limits the peripheral extent of the subarachnoid space. By the 21st postnatal day a subarachnoid space typical of the adult animal has been established.
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